Hi @gallantus, generally to accurately measured THD+N, you need need a measuring device with ~10 dB better THD+N than the device you want to measure. The best THD+N from a DAC these days is around -121 dB or so, which suggests you need a measuring device with about -130 dB or so. But, those don’t exist. So, the best you can really do is confirm it’s getting close. Next, you can use a notch to improve things further.
A good thread on attempting to measure the E70 Velvet is below. This should help you understand the limits.